1. Field of the Invention
The invention relates to presentation of media streams in response to digital content.
2. Related Art
Distribution of digital content representing media streams, such as for example movies, is subject to several problems. One problem is that it is easy to make exact copies of digital content, thus allowing any recipient of that content to redistribute it, whether authorized or not. It would be advantageous to be able to distribute digital content, particularly digital content representing media streams, without fear of its unauthorized distribution. This would be particularly advantageous when it is desired to distribute digital content using a communication link, such as for example a computer network or other technique for distribution to end viewers (for example, either on demand, in anticipation of future demand, or in response to something else).
One known solution is to encrypt the digital content that represents the media stream, so that a recipient of that digital content cannot easily redistribute it in a readily presentable (that is, unencrypted) format to unauthorized recipients. However, even when digital content is distributed in an encrypted form, it must be decrypted before it can be presented to a viewer. Thus, there is at least some time for each movie, during distribution from originator to viewer, during which that movie is available in an unencrypted format (herein sometimes also called “in the clear”). At times, and in places in any presentation system, when that movie is available in the clear, that movie is vulnerable to security attacks. For example, an unauthorized person might copy the movie in its unencrypted format and distribute or use it without authorization.
Accordingly, it would be advantageous to provide a method (and devices for performing it) by which the digital content can be used for presentation as a media stream, without exposing that digital content in the clear. However, there are several issues related to achieving this goal.                It would be desirable for the device to be relatively tamper-resistant, so that the work factor for obtaining the digital content in the clear would be substantially greater than simply purchasing copies (or at least, greater than other possibly available techniques for unauthorized procurement).        It would also be desirable for the device to expose the digital content representing the media stream as little as possible. For some examples, having the digital content (or a key from which that digital content could be obtained) in the clear in a memory would be less desirable than only having the digital content in the clear on an internal bus, which itself would be less desirable than only having the digital content in the clear when actually presented on a screen for viewing by an end-user.        
These issues present a need for separating that part of the device that has access to keys for decryption into a separate set of “trusted” hardware and software elements, with the effect that it would be advantageous for at least some of the device to be implemented in tamper-resistant hardware operating under control of verified software.                It would be desirable for the device to be able to both decode digital content representing media streams, and to provide common playback functions known for media streams, without these functions involving complete decryption of the digital content. These functions might include navigation within the digital content (such as for example fast-forward and rewind functions), content selection within the digital content (such as for example chapter-skip and multi-angle selection functions), or manipulation of the presentation (such as for example freeze-frame or single-frame-advance functions).        It would be desirable for the device to be able to provide access to metadata about the one or more media streams, such as a title or rating, or other information about the media streams for which it is generally acceptable to maintain that information in the clear, without these functions involving complete decryption of the digital content.        It would be desirable for the device to be able to provide differing access to distinct end-users for selected portions of one or more media streams, such as for example differing access to audio versus video, or English-language versus French-language versions, or US releases versus UK releases, or “airline” versions versus “general release” versions, for the same media stream, without these functions involving complete decryption of the digital content.        
It would be desirable for these playback functions, and possibly others, to be implemented in relatively unverified software. In one embodiment, only verified hardware or software would be allowed access to keys for decrypting the digital content. However, there are many such functions for which it would be desirable to have them be available to the user, without having those functions be implemented in tamper-resistant hardware (which would be more expensive, and would be difficult to update), or in verified software (which would also be more difficult to update, and might also be more expensive to create).
Formats now used for encoding digital content representing a media stream for digital distribution (such as for example MPEG-1, MPEG-2, and MPEG-4) are relatively complex. These formats provide for dividing up the digital content into multiple packets. Thus, it is possible when parsing digital content representative of media streams, that encryption might involve maintaining substantial state information across many such packets. A device able to conduct both the parsing and stitching operations might need substantial working memory. In general, having to maintain less state across packet boundaries would allow the hardware and software for decoding and decrypting the encoded and encrypted movie to be simpler, and would allow the digital content for the movie to be less exposed in the clear.
Formats used for encoding digital content representing media streams also provide for partial delivery of portions of the digital content at different times, such as when sending the digital content is interrupted and later restarted, or when packets including portions of the digital content arrive out of order, or with parts missing. Similar to the problem involving multiple packets, a device able to recover from partial delivery of only a portion of the digital content might need to maintain substantial state, or to maintain substantial working memory. In general, having to maintain less state across packet boundaries would allow the hardware and software for decoding and decrypting the encoded and encrypted movie to be more robust with regard to handling packets that arrive out of order, or with parts missing.
Formats used for encoding digital content representing media streams provide for additional information about the media stream, such as a title, for which it might be advantageous to have available even when the media stream is not actually being presented to the viewer. For example, it might be advantageous to allow a potential viewer to browse titles and related information, or even to conduct a computerized search on that information, without actually presenting the media stream. A device able to provide that information rapidly, such as on a random access basis with regard to the digital content representing that media stream, would involve substantial resources for computation and memory, likely relatively proportionate to the amount of the digital content desired to be reviewed on a random access basis, with the effect that such a device would thus be relatively insecure against attack, as either decryption keys or digital content in the clear would be available to those parts of the system for which such random access were desired.
Accordingly, it would be advantageous to provide an improved technique for presentation of digital content representing a media stream, such as the technique in which devices able to access the digital content are not allowed access to the media stream represented by that digital content, but still are allowed access to metadata regarding that media stream.